1. Field of the Invention
The present invention relates to an air intake device of a multi-cylinder engine and, more particularly, to an air intake device of an engine adapted to be supercharged by utilizing the dynamic effect of intake air obtained by the suction of air in each cylinder.
2. Description of the Prior Art
In the conventional multi-cylinder engine, an air intake device utilizing the dynamic effect of intake air for supercharging is known. Generally two types of supercharging are known: inertia supercharging and resonance supercharging (refer to U.S. Pat. Nos. 4,708,097 and 4,829,941). According to inertia supercharging, a negative pressure wave from a cylinder on an intake stroke is changed into a positive pressure wave by means of a pressure changing section represented by a surge tank, by which the intake air is forced into the cylinders for supercharging. In the meantime, in resonance supercharging, a plurality of cylinders are divided into two groups in order to prevent the continuance of the intake stroke in adjacent cylinders, and intake pipes from each group of cylinders are connected to respective intake manifold sections that are interconnected. According to this intake system, the intake air in the intake pipe is vibrated by utilizing pressure waves produced by the periodic opening and closing of the intake port, to accomplish supercharging to each cylinder by the vibratory pressure of this intake air.
In supercharging utilizing such a dynamic effect of intake air, increasing the length of a portion of the intake pipe taking part in supercharging can decrease the engine speed at which supercharging can be effected with the highest efficiency. Reversely, decreasing the length of the portion of the intake pipe can increase the engine speed at which supercharging can be performed with the highest efficiency. Therefore, to efficiently supercharge the engine within the range of both high and low engine speeds, all that has to be done is to change the length of the portion of the intake pipe that takes part in supercharging in accordance with the engine speed, or to use another portion of the intake pipe of different length according to the engine speed.
For example, there has been disclosed in Laid-Open Japanese Utility Model Application No. 61-173730 a construction that the effective pipe length of the portion of intake pipe used in supercharging can be changed according to the engine speed. That is, two intake manifold sections to which intake pipes of two groups of cylinders are connected are provided with communicating passages interconnected at each end, and the length of these communicating passages are changeable in accordance with the engine speed.
Here, it is conceivable that the arrangement of two long and short communication passages employed in place of the above-mentioned communicating passages changeable in pipe length and connected between two intake manifold sections, the long passage being used at relatively low engine speeds, and the short passage being used at high engine speeds. In this case, however, such communicating passages must be arranged properly, otherwise the intake system becomes large in size. Heretofore, however, no attention has been paid, from the point of view of realizing a compact device, to how a plurality of communicating passages should be arranged, and no material example has been suggested of such arrangement.